In MRI apparatuses, imaging of a subject is attained by irradiating the subject stayed in a uniform static magnetic field generated by a static magnetic field magnet with electromagnetic waves to induce excitation of nuclear spins in the subject, receiving electromagnetic waves generated by the nuclear spins, i.e., magnetic resonance signals, and processing the signals. The irradiation of the electromagnetic waves and reception of the magnetic resonance signals are performed by a device for transmitting and receiving electromagnetic waves of radio frequency (RF), which is called RF antenna or RF coil.
RF coils are roughly classified into two kinds of those called surface antennas or local antennas, and those called volume coils or volume antennas. The surface antennas have a round shape or tabular shape, and show sensitivity in a region near the antennas, and they are used by being put on surface of a subject. On the other hand, the volume antennas have a cylindrical shape or a shape of two discs disposed upper and lower sides, and they show sensitivity in the whole volume in the cylinder or between the discs, and used by placing a subject in that space.
Examples of the volume antennas having a cylindrical shape include those of birdcage type (refer to, for example, Non-patent document 1), and those of TEM type (refer to, for example, Patent documents 1 and 2). In these volume antennas, about 16 to 24 conductive elements of a rod shape usually called rungs (crossbars or rungs of ladder) are disposed in parallel to the center axis of the cylinder and along the side of the cylinder.
Such volume antennas having a cylindrical shape are used in MRI apparatuses of the tunnel type. In MRI apparatuses of the tunnel type, a static magnetic field magnet is disposed in a cylindrical shape to form a tunnel, a subject laid on a bed is entered into the inside of the tunnel, and imaging is performed.
Regions in which sensitivity of local antennas can be obtained often correspond to only a part of such regions of volume antennas and are narrower than such regions of volume antennas. However, since sensitivity of local antennas is usually higher than that of volume antennas, local antennas are often used as a receiving antenna. Examples of the local antennas include, for example, one consisting of a conductive element bent in the shape of a loop (refer to, for example, Patent document 3), one consisting of a conductive element bent in the shape of figure eight (refer to, for example, Non-patent document 2), and so forth.
When it is desired to image a wide region with good sensitivity, a plurality of local antennas may be disposed to extend the region in which sensitivity can be obtained, and used as if it is a multi-channel volume antenna. In such a case, the antenna is constituted as an antenna having electric power supplying and receiving terminals in a number corresponding to the channel number of 3 or more, whereas the channel number of general volume antennas is 2.